The present invention relates to an ESD protection device for protecting a circuit against electrostatic discharges (ESD) and, in particular, to an ESD protection device having a diode series circuit.
Electrostatic discharges may occur as the result of the transfer of electrical charge between a circuit and a human operator or a metallic surface, and may result in high voltages between two supply potentials of the circuit or in high currents through the circuit, which may damage or destroy the circuit. The “charged device” model (CDM) describes situations in which the circuit is charged and discharges if, for example during installation, it comes into contact with a conductive metal part. In the CDM, such discharge events are characterized by the flow of high discharge currents occurring over short time-scales, of the order of magnitude of one nanosecond. Owing to their small size, integrated semiconductor circuits are particularly sensitive to high voltages between two supply potentials of the circuit or to high discharge currents through the circuit.
To a certain extent, circuits can be protected against electrostatic discharges in that the occurrence of the latter is prevented by careful handling of the circuit during installation and operation. In addition, circuits, particularly integrated semiconductor circuits, are frequently equipped with ESD protection devices which, upon the occurrence of an electrostatic discharge, offer a low-resistance discharge path and divert the charge to one of the supply-voltage lines such that no overvoltage occurs that could damage a circuit to be protected, or such that a high discharge current does not flow through the circuit to be protected.
ESD protection devices may comprise diodes, bipolar transistors, metal-oxide transistors, thyristors or similar elements, as well as combinations of the aforementioned elements, which have a non-linear current-voltage characteristic. ESD protection devices are frequently of such design that they are high-resistance for voltages that are less than or equal to an operating voltage of the circuit to be protected while, for voltages that are significantly greater than the operating voltage of the circuit to be protected and that might damage the circuit to be protected, they enter a low-resistance state to provide a discharge path for the electrostatic discharge, said discharge path being in parallel to the circuit to be protected. In order for the circuit to be protected in an effective manner against transient discharges, particularly in the CDM, the transition from the high-resistance state to the low-resistance state of the ESD protection device should be effected over as short a time scale as possible, comparable to the characteristic time scale of the electrostatic discharge.
A conventional ESD protection device comprises a series circuit of diodes which is to be so arranged as a voltage clamp between a first supply potential, e.g. VDD, and a second supply potential, e.g. VSS, that it is connected in parallel to the circuit to be protected. However, the time required to discharge electrostatic transients using a series circuit of diodes can often be inadequate to protect the circuit. Further, the clamping characteristics of such a circuit may also be inadequate for certain implementations.
Therefore, there is a need for an ESD protection device, in which the time required from the transition from a high-resistance to a low-resistance state is reduced in comparison with the conventional ESD protection device having a diode series circuit. Furthermore, there is a need for an ESD protection device which, compared with the conventional ESD protection device having a diode series circuit, has comparable or improved clamping characteristics in combination with an improved time characteristics.